Vehicle emissions are highly regulated to minimize the output of environmentally-harmful exhaust emissions. The major regulated pollutants include carbon monoxide (CO), nitrogen oxide compounds (NOx), and unburned hydrocarbons (CxHy). If the vehicle exhaust is left untreated, the levels of pollutants would far exceed the emissions standards set by, for example, the U.S. Environmental Protection Agency, the states, or another country.
To meet these standards, vehicles include exhaust aftertreatment systems that include catalytic converters, such as three-way catalytic (TWC) converters, to convert gaseous CO, NOx, and CxHy into less harmful compounds through oxidation and reduction reactions. An example of such an exhaust aftertreatment system is illustrated in FIG. 1, which is a block diagram of an underbody of a vehicle 10. The vehicle 10 includes engine 100, first catalytic converter 110, second catalytic converter 120, and muffler 130, which are in fluid communication with one another through pipe or conduit 140. In operation, the engine 100 generates exhaust, which travels through conduit 140 to first catalytic converter 110, second catalytic converter 120, muffler 130, and then into the environment through tail pipe 150.
Recently, emissions regulators have become increasingly concerned about particulate emissions and setting limits on their levels in engine exhausts both in terms of their total mass (PM) and number (PN). These particulates are generated inside internal combustion engines in three basic forms: (1) condensables (also referred to as PM2.5 when their size is less than 2.5 microns), (2) pure solids, generally referred to as “black carbon,” and (3) carbon particles saturated with volatile hydrocarbon condensables, generally referred to as semivolatile particles or “brown carbon.” At the high temperatures typical inside a standard exhaust aftertreatment system, such as that illustrated in FIG. 1, some these particulates form into liquid-phase and solid-phase particulates before the exhaust gases reach the tailpipe, while some of the volatile hydrocarbon condensables remain in their gaseous phase. After exiting the tailpipe, volatile hydrocarbon condensables cool and return to the liquid phase, appearing as an aerosol. The final state of the condensables depends on the temperature, degree of dilution, other particulates in the atmosphere, etc.
Gasoline particulate filters (GPFs) and catalyzed gasoline particulate filters (cGPFs), coupled in some form to a catalytic converter, have been proposed for removing particulates from hot exhaust gases before they exit the tailpipe. However, GPFs and cGPFs cannot remove volatile hydrocarbon condensables in their gaseous form. In addition to exiting the exhaust system as a liquid (e.g., as an aerosol), gaseous volatile hydrocarbon condensables can form additional particulates downstream of the GPF/cGPF, for example in the muffler or as they exit the tail pipe.
An additional problem with existing exhaust aftertreatment systems is that the high operating temperatures of the second catalytic converter 120 cause NOx to reform, which is undesirable and, in some instances, prevents the vehicle from complying with emissions regulations.
It would be desirable to overcome one or more of the foregoing problems.